More STP

The question went somehting like: "You have just pulled new switches out of the box, connected the cables, and applied power. The switches immediately begin to send out BPDU's to elect a root bridge. During this time what state are the ports in?"

Forwarding
Blocking
learning
listening

The explanation to the question was that the answer is Learning and of course I chose listening.

I understand that the stp ports transition by Blocking, listening, learning, forwarding.

But i thought learning was to learn MAC addresses not root. I thought listening state listens for better BPDU's. So am I wrong or is this another bunk question?

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Comments

markzab

Without actually going to google and looking it up, if I'm not mistaken, isn't the listening state when the port is accepting information but not forwarding it? To put it simply.

mikearama

Sounds like a crap question to me. You're right... that description sounds exactly like Listening. Listening is when a switch wants to be the Root, and decide on root/designated ports.

And Learning has nothing to do with root election, but as you suggested, mac table building.

markzab

Nah, I was wrong...but this is interesting...

http://www.cisco.com/univercd/cc/td/doc/product/rtrmgmt/sw_ntman/cwsimain/cwsi2/cwsiug2/vlan2/stpapp.htm#xtocid69355

A port moves through these five states as follows:

From initialization to blocking

From blocking to listening or to disabled

From listening to learning or to disabled

From learning to forwarding or to disabled

From forwarding to disabled

So shouldn't the answer to the question actually be blocking?

Actually, if you read further on that page it notes that once BPDU's are sent the switch would enter into listening state. So the original question is a little tricky if you didnt take note of it saying BPDU's have already been sent.

Netstudent

my thoughts exactly.

And yes Mark, without any prior knowledge on STP it would be easy to choose learning because it just seems logically correct that the switches are learning about the better priority bridges.
But i don't think that is the case. So unless someone who knows for sure, or can provide information stating otherwise, I guess I'm calling bull **** on that one.....

I believe the question was wanting to know the state during the election or during the process of sending out BPDU's.

markzab

mikearama wrote:

Sounds like a crap question to me. You're right... that description sounds exactly like Listening. Listening is when a switch wants to be the Root, and decide on root/designated ports.

And Learning has nothing to do with root election, but as you suggested, mac table building.

The question is tricky because it isn't asking what state it is in when it's selecting a root bridge. It's more asking what state the ports are in once BPDU's have been sent (and then it is just noting that BPDU's are sent when electing root ports/bridges).

Netstudent

tricky indeed. But either way, I don;t see how it could be learning. When learning is all about learning the MAC entries in relation to the new or changed spanning-tree.

mikearama

From the CCNP BCMSN Official Exam Certification Guide (cisco press) by David Hucaby, page 197...

Listening: A port is moved from Blocking to Listening if the switch thinks that the port can be selected as a Root Port or Designated Port.

Learning: The port still sends and receives BPDU's as before. In addition, the switch now can learn new mac addresses to add to its address table.

I don't see how it can be learning either... I'd have gone with Listening all day long.

Mike

markzab

Netstudent wrote:

tricky indeed. But either way, I don;t see how it could be learning. When learning is all about learning the MAC entries in relation to the new or changed spanning-tree.

No, I agree. I was wrong in my first post. That's why I posted that info from cisco's site. The port goes into listending from blocking once BPDUs are sent out. You were right, the question was wrong.

Netstudent

okay thanks guys

Stotic

Where'd this question come from?

Netstudent

uhhhhhh I made it up. BUT the cisco press ICND cd may have had soemthing that sparked the question.

mikearama

I think NS is making these questions up... keep us on our toes.

He's sneaky that way.

EDIT: See, told ya so.

Netstudent

I do like to spark a good discussion with good questions.

Stotic

Oh, the question being quoted led me to believe that it came from another source.

Netstudent

it was actually paraphrased. REALLY paraphrased!!! I didn't want the post to get nuked because of copyright issues. I'v been kinda prone to that lately because of STP.